Electronic assemblies are comprised of many devices in the larger or main frame computers, including logic and memory chips, which are attached to common chip support substrates. A chip support substrate carries on its surface the necessary circuit patterns for conducting signals to and from the chips mounted onto the substrate. In addition to the circuit patterns on the substrate surface, also there may exist a plurality of attachment points or pads formed onto the surface of the substrate. These pads permit soldering the termination pads of the logic and/or memory chips to the electrical circuit patterns on the chip support substrate.
Typically, this attachment of the logic and memory chips to the substrate is accomplished by small solder balls formed on the termination pads of the logic or memory chip. This attachment technique is well known as a part of the C4 solder process. Once the substantially uniform diameter solder balls have been formed on the termination pads of the electronic chip, this chip may be placed over the appropriate termination pads of the electrical pattern on the substrate and the solder reflowed to make the connection and bridge between the termination pads on the chip and the chip support substrate.
When the solder balls initially are formed on the electronic chips or the chip support substrate, there will be inherent variations in the amount of solder in each of the solder balls, notwithstanding the best efforts to maintain uniformity. In addition, the thickness of the several chips will vary slightly since the chips are made from different silicon slices. These variations cause problems in cooling of each chip, because the top surface of all the chips will not be uniformly co-planar.
Thus, to a very large extent, the orientation of the chip with respect to the chip support substrate is controlled by the surface tension of the liquid solder during solder reflow and by the volume of solder of each solder ball. Accordingly, the height of the exposed surface of the electronic chip from and the orientation of the exposed surface of the chip with respect to the top surface or the circuit surface of the chip support substrate is largely dependent upon the solder surface tension and the volume of solder of each ball.
Additionally, there may exist variations in the thickness of the chip itself from one edge to the other; these variations will influence the orientation of the top or exposed surface of the electronic chip relative to the circuit surface of the chip support substrate. With all these variables, in all probability, the exposed or top surface of the electronic chip will not be parallel and/or at a uniform height with all the other surrounding chips relative to the circuit surface of the chip support substrate.
It has also been found that it is virtually impossible to reliably orient all the exposed or top chip surfaces so that they are parallel to the exposed circuit surface of the chip support substrate.
As a result of this non-uniformity, the use of a single flat plate to cool the chips during operation is hampered by the inability to place a single planar surface over a plurality of the chips and accomplish intimate, surface-to-surface contact with all the chips.
As the circuit density within the chips increases, a result of the placement of larger numbers of integrated circuits onto the electronic chips, more heat is generated and concentrated in the chips. The heat must be efficiently and reliably removed from the chips in order to maintain the chips in an efficient operating condition.
To accomplish this heat removal, thermal conduction cooling modules have been designed to engage the exposed surface of the electronic chips with members which are highly heat conductive. The members are contained within a frame or framework which biases the members against the surface of the chips. This framework also provides a thermally conductive path to carry the heat away from the electronic chips, thereby cooling the chips.
Contact between the engaging member and the chip becomes either a point contact or a line contact. If the engaging member contacts the chip's exposed surface, has a planar surface, is constrained to move only in an axial direction generally perpendicular to the plane of the chip and if the chip is tilted slightly, the heat transfer through this engagement is seriously degraded from that desired and, accordingly, efforts have been made in the past to overcome the effect of tilted chips. Early attempts at conforming the heat conductive member to the tilted orientation of the chips have included: the assembly of the chip and its package with a relatively large mass of low melting point solder positioned between the package and the exposed surface of the electronic chip, illustrated in U.S. Pat. No. 4,034,468 to Koopman, and commonly assigned with this application. After complete assembly of the package and electronic chip into a module, the solder then is reflowed and allowed to "ball up" due to surface tension and gravity; the effect being that the ball of solder then will sag and come into contact with either the chip or the package, depending upon the orientation of the elements of the module and the pre-reflow position of the mass of solder. At least one of the junctions between the solder and one of its adjacent engaging surfaces is not a bonded interface; accordingly, when cryogenic cooling is used, the varying coefficient of thermal expansion of the different materials may degrade the heat transfer across the unbonded surface or create stresses on the chips, leading to breakage.
One approach to permanent thermal connections of the electronic devices to the heat dissipating members includes the use of solders to metallurgically bond the electronic device to the heat sink, such as is described in the IBM Technical Disclosure Bulletin entitled "Chip Heat Sink Package Assembly" by A. A. Johnson, et. al., Volume 12, No. 10, March 1970, page 1665. Additionally, the use of a liquid gallium heat transfer layer in a circuit module is disclosed in IBM Technical Disclosure Bulletin, Volume 19, No. 4, September 1976, page 1348, entitled "Circuit Module With Gallium Metal Cooling Structure" by D. A. Jeannotte.
Later attempts to design conduction cooling modules to cool a plurality of electronic chips have resulted in the use of compliant pistons contained within a frame each within its own cylinder and biased against the electronic chip, as illustrated in U.S. Pat. No. 4,193,445 to Richard C. Chu, et. al., and commonly assigned with this application.
A further effort to enhance heat transfer from the chip to the cold plate resulted in pistons shaped with a slight taper on both ends to allow tilting of the piston within the cylinder of the cold plate. This addressed the piston/chip interface, but did not solve the gap/barrier to heat conduction from the piston to the cold plate. Filling the gap with an oil helps but is still inefficient. The tapering of the piston allows the piston to tilt or shift in a cylinder which is more nearly the same diameter as the piston, reducing the gap and enhancing heat transfer across the gap.
The pistons and cold plate cylinders typically require close tolerances to have satisfactory heat conductivity. A gap between the piston and cylinder generally must be not greater than 0.001 inches or 0.025 mm, in this environment.
U.S. Pat. No. 4,193,445 addresses the problem of tilted electronic chips by formation of the head or engaging surface of a spring biased piston into a partially spherical shape whereby the partially spherical surface of the piston will engage the exposed surface of the electronic chip regardless of the chip tilt or orientation. The piston of this patent is provided with a central bore extending to a channel arrangement on the spherical face, whereby a wax having thermally conductive properties may be loaded into the central bore of the piston. When heated, the wax will flow downward toward the face of the chip effectively providing a bond between the piston and the chip, thereby filling the area between the piston face and the exposed surface of the chip to enhance thermal conductivity from the chip to the piston. The annular interface between the piston and the cylinder wall likewise is filled with the melted thermal wax, upon reflow, to prevent rebound of the piston from the chip face and to enhance thermal conductivity to the cylinder walls.